/OSI Model

OSI Model

Open System Interconnection Model (AKA OSI model). it’s a way of sub-dividing communications system into small parts called layers .A layer is a collection of similar functions that provide services to the layer above it and receives services from the layer below it (which i’ll talk about them in brief in this article)

There are 7 layers in the OSI model :

1- Physical layer : The physical layer defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or optical cable. This includes the layout of pins, voltages, cable specifications and more .

2- Data link layer : Provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system .

3- Network layer : Provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network, while maintaining the quality of service requested by the Transport Layer (in contrast to the data link layer which connects hosts within the same network). The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. routers operate at this layer , sending data throughout the extended network and making the Internet possible .

4- Transport Layer : Provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail. The Transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred.

5- Session layer : Controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures . The Session Layer is commonly implemented explicitly in application environments that use ” remote procedure calls ” .

6- Presentation layer : Establishes context between Application Layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack.This layer provides independence from data representation (like : encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called ” the syntax layer ” .

7- Application layer : The OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.

Here’s a Picture of the 7 layers of the OSI 

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